Solution or lacquer adhesives are utilized in a wide variety of applications. Such adhesives are particularly useful because they can be easily applied to a substrate.
Solution adhesives are bonding compositions which achieve a solid state and resultant strength through evaporation or removal of solvent. This is in contrast to hot melt adhesives which achieve a solid state and resultant strength by cooling from an elevated temperature. In other words, the application of heat melts the hot melt adhesive and after the heat source is removed, it solidifies simply by cooling. Hot melt adhesives are widely used in industrial applications because they can be used to form strong bonds very quickly. However, the difficulty associated with applying hot melt adhesives at an elevated temperature has greatly limited their usage in many applications.
Solution adhesives offer an advantage over hot melt adhesives in that heating is not required in their application to a substrate. Solution adhesives are widely utilized in general purpose and household applications because of their ease of application. Solution adhesives are also the adhesive of choice in a wide variety of industrial applications, such as those where the application of a hot melt adhesive would be difficult or impractical.
Copolyester resins are frequently utilized in solution adhesive compositions. Such solution adhesive compositions are simply prepared by dissolving the copolyester resin in a suitable organic solvent. It is highly desirable for the organic solvent to have a low boiling point. This results in faster rates of solvent evaporation and consequently reduces the time required for bonding. The copolyester resins utilized in solution adhesives commonly contain repeat units which are derived from isophthalic acid. For example, polyethylene terephthalate/isophthalate/azelate copolyesters are widely utilized as solution adhesive resins. Unfortunately, solutions of such copolyester resins form visible haze on standing. Haze formation in such solutions is, of course, highly undesirable because it renders the solution adhesive composition unsuitable for use in many applications.
The haze which forms in such solution adhesives is due to the presence of cyclic oligomers, such as the cyclic dimer of ethylene azelate, the cyclic dimer of ethylene isophthalate, the cyclic monomer of ethylene azelate, and the cyclic trimer of ethylene terephthalate. The cyclic dimer of ethylene isophthalate is believed to be the predominant cyclic oligomer in such solutions. It is the result of the cyclization of ethylene glycol and isophthalic acid to form the following ring structure: ##STR1##
This ring structure contains two units of ethylene isophthalate (hence, the name cyclic dimer) and is a by-product of the polyesterification reaction of ethylene glycol with isophthalic acid or of the transesterification reaction of ethylene glycol with isophthalate esters such as dimethyl- or diethylisophthalate.
Cyclic dimer is produced as an unwanted reaction by-product in conventional synthesis techniques for preparing copolyester resins which contain repeat units which are derived from isophthalic acid. In such conventional synthesis techniques, the cyclic dimer is believed to be formed during the esterification stage of the reaction. This is because cyclic dimer is the esterification product of two isophthalic acid molecules with two ethylene glycol molecules.